Evaluation of highway bridges is a matter of importance in terms of seismic performance analysis for pre-and post-earthquake planning of a transportation system. Irregular structures have always been in the spotlight of researchers and engineers. In this study, a three-dimensional model of seat-type abutments is generated in a set of irregular bridges with unequal height of piers using OpenSess finite element software. Since altitudinal irregularity leads to different stiffness of individual piers, the distribution of seismic forces will be dissimilar for each pier. On the other hand, the proportion of seismic forces absorbed by the abutments is dependent on strength and stiffness of central piers. In addition to irregularity effects, the influence of two distinct approaches of pier support modeling such as fixed-base and flexible-base (soil-structure-interaction) is taken into account in the analyses. In addition, in order to obtain maximum demand on bridge members with complex behavior, subjected to the ground motion by orthogonal components, nonlinear time-history analysis using multiple earthquake records should be applied in different directions. Therefore, incremental dynamic analysis is performed on each bridge model for a set of seismic records each rotated in seven various directions. By processing the outcomes obtained from analysis of two levels of damage states, it is determined that the irregularity ratio and configuration, soil-structure-interaction, and incident angle of seismic motions are three important factors in evaluation of fragility characteristics of abutment constitutive members. With no exceptions in the investigated models, the fixed-base assumption of central piers produces conservative response of the abutment components in comparison to soil-structure-interaction consideration. However, the effects of ground motion directionality on the fragility characteristics of individual members of abutment vary for different damage states from one model to another.